Light redirecting films are typically thin transparent optical films or substrates that redistribute the light passing through the films such that the distribution of the light exiting the films is directed more normal to the surface of the films. Typically, redirecting films are provided with ordered prismatic grooves, lenticular grooves, or pyramids on the light exit surface of the films which change the angle of the film/air interface for light rays exiting the films and cause the components of the incident light distribution traveling in a plane perpendicular to the refracting surfaces of the grooves to be redistributed in a direction more normal to the surface of the films. Such light redirecting films are used, for example, to improve brightness in liquid crystal displays (LCD), laptop computers, word processors, avionic displays, cell phones, PDAs and the like to make the displays brighter.
Previous light redirecting films suffer from visible moiré patterns when the light redirecting film is used with a liquid crystal or other display. The surface features of the light redirecting film interact with other optical films utilized in backlight assemblies, the pattern of printed dots or three-dimensional features on the back of the light guide plate, or the pixel pattern inside the liquid crystal section of the display to create moiré, an undesirable effect. Methods known in the art for reducing moiré have been to die cut the light redirecting films such that the lenticular array, is not parallel to any edge of the sheet, such that the lenticular array is at an angle relative to another light redirecting film or to the display electronics. Methods also used include randomizing the linear array by widths of the linear array elements, varying the height of the grooves in the linear array, adding a diffusing surface to the film on the side opposite the linear array, adding a diffusing film to the system, or rounding the ridges of the linear array. The above techniques to reduce moiré also cause a decrease in on-axis brightness or do not work to adequately solve the moiré problem. Moire and on-axis brightness tend to be related, meaning that a film with high on-axis gain would have high moiré in a system. It would be beneficial to be able to reduce the moiré while maintaining relatively high on-axis gain.
U.S. Pat. No. 5,919,551 (Cobb, Jr. et al) discloses a linear array film with variable pitch ridges and/or grooves to reduce the visibility of moiré interference patterns. The pitch variations can be over groups of adjacent ridges and/or valleys or between adjacent pairs of ridges and/or valleys. While this varying of the pitch of the linear array elements reduces moiré, the linear elements of the film still interact with the dot pattern on the backlight light guide and the electronics inside the liquid crystal section of the display. It would be desirable to break up the linear array of elements to reduce or eliminate this interaction.
U.S. Pat. No. 6,354,709 discloses a film with a linear array that varies in height along its ridgeline and the ridgeline also moves side to side. While the film does redirect light and its varying height along the ridgeline slightly reduces moiré, it would be desirable to have a film that significantly reduces the moiré of the film when used in a system while maintaining a moderately high on-axis gain.
U.S. Pat. No. 6,583,936 (Kaminsky et al) discloses a patterned roller for the micro-replication of light polymer diffusion lenses. The patterned roller is created by first bead blasting the roller with multiple sized particles, followed by a chroming process that creates micro-nodules. The manufacturing method for the roller is well suited for light diffusion lenses that are intended to diffuse incident light energy.